The gastropod Nucella lapillus (L.) is abundant between tidemarks on many rocky shores, where it feeds mainly on mussels (Mytilus edulis L.) and on barnacles. The form of the shell varies (Moore 1936); on the open coast at Carrigathorna, near Lough Ine, the shells are wider and thinner than in the Lough Ine Rapids (Ebling, Kitching, Muntz & Taylor 1964). We do not know to what extent these differences are determined by environmental conditions, such as nutrition or wave action, and to what extent they are of genetic origin. Moore (1936) noted that the thin-shelled forms tend to feed on mussels. On the other hand, Staiger (1957) has described gross differences in the chromosomes of populations of Nucella from different habitats near Roscoff.